The present invention relates to an adaptive equalization system of nonlinearities in a pre-distortion nonlinearity compensation system and/or a post-distortion nonlinearity compensation system, in particular, it relates to such a system which performs automatically the compensation of the nonlinearities of a circuit.
The present adaptive equalization system is preferably applicable to compensate the nonlinearities of a traveling-wave tube amplifier.
A prior equalization system for the compensation of the nonlinearities for a high frequency region in which a negative feed-back system can not be utilized because of the delay and/or the phase characteristics of the feedback loop, is a pre-distortion system, or a post-distortion system.
However, said prior equalization systems have the disadvantage that the equalization operation is not always complete since the structure and the operation of the equalization system are independent from that of a circuit to be equalized, that is to say, when the characteristics of one of the equalizer and the circuit to be equalized fluctuate due to the change of the ambient temperature or a secular variation, an undesirable residual nonlinearities will be generated.
In order to solve that problem, a system which maintain the gain of a circuit to be equalized constant has been proposed (for instance, Japanese patent laid open publication No. 54-107656). However, that proposal is effective only when the change of the nonlinearities of both the amplitude and the phase are small, and the change of the absolute value of the amplitude is large. Accordingly, when the nonlinearities themselves change, like the case when the power source of a circuit to be equalized changes, said proposal can not be utilized.
Therefore, a prior equalization system must be readjusted frequently to provide the desired characteristics.